Viscous food products such as cake decorations and soft cheese are presently packed in valved pressure dispensing cans on top of barrier pistons, which separate the product from a pressurizing gas beneath the piston. Some such cans have side seams which present inner surface irregularities; even unseamed cans may be bent slightly out-of-round. The cylindrical wall pistons heretofore in use, molded of a flexible plastic such as polyethylene, do not yield sufficiently to make up for such can irregularities; though molded with projecting circumferential rubbing beads.